This invention relates to power supply circuits, and more particularly to a high-efficiency switching regulator circuit.
When a portable computer is operated on battery power, low power consumption becomes important so that the size and weight of the batteries required to provide the desired running time will be reduced. Also, since it is preferable to omit a fan in the design of battery powered equipment, high efficiency is needed to avoid high temperatures in power supply components.
Synchronous rectifier circuits are widely used for reducing losses due to voltage drop across a rectifier. Bipolar and MOS transistors are available which provide synchronous rectification with rectifier drops of less than about 0.25 V, allowing an efficiency of rectification of greater than 90% for a 5 V power supply output, as would typically be used in a portable computer; this can result in an overall DC-to-DC efficiency of greater than 80% in a switching converter/regulator. A switching regulator using low-voltage MOSpower transistors is show in an article by James Blanc, Interface, published by Siliconix Incorporated, Winter 1987, pp. 20-21; this circuit is especially useful as a power supply for a low-voltage digital IC system, where the forward drop of more conventional rectifiers would be a larger fraction of the desired output voltage. However, it is desirable to improve this efficiency to reduce the heat generated in the power supply components and to reduce the battery size and weight. One of the main sources of switching losses is improper timing of the drive signals to the rectifier transistors; if both rectifiers are on simultaneously in a typical circuit, the transformer winding is effectively shorted, resulting in power losses and high voltage spikes from energy stored in transformer leakage inductance.
It is therefore the principal object of this invention to provide an improved power supply circuit for use in a portable computer or the like. Another object is to provide a high-efficiency synchronous rectifier circuit. A further object is to provide a method of controlling the gates of power FETs in a switching regulator to prevent current spikes at transitions. An additional object is to provide a power supply circuit for a small portable electronic device, particularly a circuit which will allow the use of minimum-sized batteries and will generate a minimum of heat so that a cooling fan will not be needed.